And dependent fixes and misc cleanup.

Co-hacking with josharian at Gophercon.

Change-Id: Ib85dc13b303929017eb0a4d2fc2f603485f7479b
Reviewed-on: https://go-review.googlesource.com/12027Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>